CN112201205B - Method and system for equalizing pixel circuits - Google Patents

Abstract

The invention relates to a method for equalizing a pixel circuit in an array of pixel circuits, the pixel circuit comprising semiconductor devices that age differently under different environmental and stress conditions, the method comprising: a) During a period of use, creating a stress history for the pixel circuit, the stress history comprising data relating to a history of stresses to which the pixel circuit has been subjected; b) Extracting pixel parameters from the pixel circuit array during or after the use period; c) Updating a stress pattern of the pixel circuit array based on the extracted pixel parameters and the stress history; d) Subjecting the pixel circuit to stress according to the stress pattern; e) Extracting pixel parameters from the stressed pixel circuit; f) Determining whether the extracted pixel parameters are within a preselected range; when it is not within the pre-selected range: updating the stress history and repeating steps c) to f); when it is within the pre-selected range, the pixel circuit array is returned to normal operation.

Description

Method and system for equalizing pixel circuits

The present application is a divisional application of the patent application No. 201610007145.3 filed on 2016, 6.1.2016 entitled "system and method for extracting correlation curves of organic light emitting device".

Technical Field

The present invention relates generally to displays using light emitting devices such as OLEDs, and more particularly to extracting characterization correlation curves under different stress conditions in such displays in order to compensate for aging of the light emitting devices.

Background

Active Matrix Organic Light Emitting Device (AMOLED) displays offer the advantages of lower power consumption, manufacturing flexibility, and faster refresh rates relative to conventional liquid crystal displays. In contrast to conventional liquid crystal displays, there is no backlight in AMOLED displays because each pixel is composed of different color OLEDs that emit light independently. The OLED emits light based on a current supplied by the driving transistor. The driving transistor is typically a Thin Film Transistor (TFT). The power consumption of each pixel has a direct relationship to the amount of light generated in that pixel.

During operation of the organic light emitting diode device, it suffers from degradation, which results in a decrease in light output at constant current over time. OLED devices also suffer from electrical degradation, which results in a decrease in current at constant bias voltage over time. These degradations are essentially caused by the stresses related to the magnitude and duration of the applied voltage across the OLED and thus the current generated in the device. Such degradation is compounded by the contribution over time of environmental factors such as temperature, humidity, or the presence of oxidizing agents. The aging rate of thin film transistor devices is also dependent on the environment and stress (bias). The pixel is calibrated against the pixel history data stored several previous times to determine the aging effect on the pixel so that the aging of the pixel transistor and OLED can be properly determined. Therefore, accurate aging data is required throughout the lifetime of the display device.

In one OLED display compensation technique, the aging (and/or uniformity) of the pixel panel is extracted and stored as raw or processed data in a look-up table. The compensation module then uses the stored data to compensate for any shifts in the electrical and optical parameters of the OLED (e.g., shifts in OLED operating voltage and optical efficiency) and any shifts in the electrical and optical parameters of the backplane (e.g., shifts in the threshold voltage of the TFTs), thus modifying the programming voltage of each pixel according to the stored data and video content. The compensation module modifies the bias of the drive TFT in such a way that sufficient current is passed through the OLED to maintain the same brightness level for each gray level. In other words, the proper programming voltage properly offsets electrical and optical aging of the OLED and electrical degradation of the TFT.

The electrical parameters of the backplane TFT and OLED devices are continuously monitored and extracted by a measurement circuit based on electrical feedback during the lifetime of the display. Further, an optical aging parameter of the OLED device is estimated from the electrical degradation data of the OLED. However, the optical aging effect of OLEDs also depends on the stress conditions on the individual pixels, and since the stress varies from pixel to pixel, an accurate compensation cannot be ensured unless a compensation suitable for a specific stress level is determined.

Therefore, for stress conditions on the active pixels, it is necessary to efficiently extract a characteristic correlation curve of precise optical and electrical parameters for compensating aging effects and other effects. It is desirable to have various characteristic dependence curves for the various stress conditions that an active pixel may experience during operation of the display. An accurate compensation system is also required for the pixels in displays based on organic light emitting devices.

Disclosure of Invention

According to one embodiment, a system is provided for equalizing pixels in an array of pixels, the pixels including semiconductor devices that age differently under different environmental and stress conditions. The system comprises the following steps: extracting at least one pixel parameter from the array; creating a stress pattern for the array based on the extracted pixel parameters; subjecting the pixels to stress according to the stress pattern; extracting the pixel parameters from the stressed pixels; determining whether the pixel parameter extracted from the stressed pixel is within a preselected range, and when the answer is negative: creating another stress pattern for the array based on the pixel parameters extracted from the stressed pixels; subjecting the pixel to stress according to the further stress pattern; extracting the pixel parameters from the stressed pixels; and determining whether the pixel parameter extracted from the stressed pixel is within the preselected range. When the answer is affirmative, returning the pixel array to normal operation.

In another embodiment, a system for equalizing pixels in an array of pixels, the pixels comprising semiconductor devices that age differently under different environmental and stress conditions, the system: creating a stress history for the pixel during a use period; extracting at least one pixel parameter from the array after the period of use; creating a stress pattern for the array based on the extracted pixel parameters; subjecting the pixels to stress according to the stress pattern; extracting the pixel parameters from the stressed pixels; determining whether the pixel parameter extracted from the stressed pixel is within a preselected range, and when the answer is negative: creating another stress pattern for the array based on the pixel parameters extracted from the stressed pixels; subjecting the pixel to stress according to the further stress pattern; extracting the pixel parameters from the stressed pixels; and determining whether the pixel parameter extracted from the stressed pixel is within the preselected range. When the answer is affirmative, returning the pixel array to normal operation.

Various aspects of the invention will become apparent to those skilled in the art in view of the detailed description of various embodiments, which proceeds with reference to the accompanying drawings, a brief description of which is provided below.

Drawings

The invention may best be understood by referring to the following description taken in conjunction with the accompanying drawings.

FIG. 1 is a block diagram of an AMOLED display system with compensation control.

FIG. 2 is a circuit diagram of one of the reference pixels in FIG. 1 for modifying a characterization correlation curve based on measurement data.

Fig. 3 is a graph of luminance emitted from an active pixel reflecting different levels of stress conditions over time that may require different compensation.

FIG. 4 is a graph of different characteristic correlation curves and results of a technique for determining compensation using predetermined stress conditions.

Fig. 5 is a flow chart of a process for determining and updating a characterization correlation curve based on a reference pixel set under predetermined stress conditions.

FIG. 6 is a flow chart of a process for compensating a programming voltage of an active pixel on a display using a predetermined characterization correlation curve.

Fig. 7 is a graph of the correlation of OLED efficiency degradation with changes in OLED voltage.

FIG. 8 is a graph of OLED stress history versus stress intensity.

FIG. 9A is a graph of OLED voltage change versus time under different stress conditions.

FIG. 9B is a graph of OLED voltage rate of change versus time under different stress conditions.

FIG. 10 is a graph of OLED voltage change rate versus OLED voltage change under different stress conditions.

FIG. 11 is a flow chart of a process of extracting OLED efficiency degradation from changes in OLED parameters such as OLED voltage.

FIG. 12 is an OLED correlation curve for OLED electrical signal and efficiency degradation.

FIG. 13 is a flow chart of a process of extracting a correlation curve from a test device.

FIG. 14 is a flow chart of a process of computing a correlation curve from a library.

Fig. 15A and 15B are flow diagrams of a process for identifying a stress condition of a device based on a rate of change or an absolute value of a parameter of the device or another device.

Fig. 16 is an example of the IV characteristics of an OLED subjected to three different stress conditions.

FIG. 17 is a flow chart of a process for achieving initial equalization of pixels in a light emitting display.

FIG. 18 is a flow chart of a process for achieving equalization of pixels in a light emitting display after a period of use.

While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and have been described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

Detailed Description

Fig. 1 is an electronic display system 100 having an active matrix area or pixel array 102 in which an array of active pixels 104 are arranged in a row and column configuration in the active matrix area or pixel array 102. For ease of illustration, only two rows and columns are shown. Outside the active matrix area (pixel array 102) is a peripheral area 106, in which peripheral area 106 peripheral circuits for driving and controlling the area of the pixel array 102 are arranged. The peripheral circuits include a gate or address driver circuit 108, a source or data driver circuit 110, a controller 112, and an optional voltage source (e.g., EL _ Vdd) driver 114. The controller 112 controls the gate driver 108, the source driver 110, and the voltage source driver 114. Under the control of the controller 112, the gate driver 108 operates on address or select lines SEL [ i ], SEL [ i +1], etc., where there is one address or select line in each row of pixels 104 in the pixel array 102. In the pixel sharing configuration described below, gate or address driver circuit 108 may also optionally operate on global select lines GSEL [ j ] and optionally/GSEL [ j ], which operate on multiple rows of pixels 104 in pixel array 102 (e.g., every second row of pixels 104). Under the control of the controller 112, the source driver circuit 110 operates on voltage data lines Vdata [ k ], vdata [ k +1], and the like, one for each column of pixels 104 in the pixel array 102. The voltage data line carries to each pixel 104 voltage programming information representing the brightness of each light emitting device in the pixel 104. The storage element (e.g., capacitor) in each pixel 104 stores voltage programming information until a light emitting device is turned on for a light emitting or driving cycle. Under the control of the controller 112, an optional voltage source driver 114 controls a voltage source (EL _ Vdd) line, where there is one voltage source line in each row of pixels 104 in the pixel array 102. The controller 112 is also coupled to a memory 118, the memory 118 being used to store various characterization correlation curves for the pixels 104 and aging parameters as will be described below. The memory 118 may be one or more of flash memory, SRAM, DRAM, combinations thereof, and/or other memory.

The display system 100 may also include a current source circuit that provides a fixed current on the current bias line. In some configurations, a reference current can be provided to the current source circuit. In such a configuration, the current source control section controls the timing of applying the bias current on the current bias line. In a configuration where no reference current is provided to the current source circuit, the current source address driver controls the timing of applying the bias current on the current bias line.

It is known that for each pixel 104 in the display system 100, it is necessary to program it with information representing the brightness of the light emitting device in that pixel 104. The frame defines a time period including a programming period or phase during which each pixel in the display system 100 is programmed with a programming voltage representing a brightness and a driving or light emitting period or phase during which each light emitting device in each pixel is turned on to emit light at a brightness corresponding to the programming voltage stored in the storage element. Thus, a frame is one of many still images that make up the complete motion picture displayed on the display system 100. There are at least two schemes for programming and driving pixels: line by line or frame by frame. In row-by-row programming, one row of pixels is programmed and then driven before the next row of pixels is programmed and driven. In frame-by-frame programming, all rows of pixels in the display system 100 are programmed first, and all frames are driven row-by-row. Either scheme may employ a brief vertical blanking time at the beginning or end of each period during which the pixel is not programmed or driven.

Components located outside of pixel array 102 may be arranged in a peripheral region 106 around pixel array 102, peripheral region 106 being arranged on the same physical substrate as pixel array 102. These components include a gate driver 108, a source driver 110, and an optional voltage source control 114. Alternatively, some components in the peripheral region may be arranged on the same substrate as the pixel array 102 while other components are arranged on a different substrate, or all components in the peripheral region may be arranged on a different substrate from the pixel array 102. The gate driver 108, the source driver 110, and the voltage source control section 114 together constitute a display driver circuit. The display driver circuitry in some configurations may include the gate driver 108 and the source driver 110 but not the voltage source control 114.

The display system 100 also includes a current source and read circuit 120, the current source and read circuit 120 reading output data from data output lines VD [ k ], VD [ k +1], etc., where there is one data output line in each column of active pixels 104 in the pixel array 102. Groups of optional reference devices (e.g., reference pixels) 130 are fabricated in the peripheral region 106 and arranged on the edges of the pixel array 102 outside of the active pixels 104. The reference pixels 130 may also receive input signals from the controller 112 and may output data signals to the current source and read circuit 120. The reference pixel 130 includes a driving transistor and an OLED, but is not part of the pixel array 102 for displaying an image. As will be explained below, different sets of reference pixels 130 are in different stress conditions via different current levels from the current supply circuit 120. Since the reference pixels 130 are not part of the pixel array 102 and therefore do not display an image, the reference pixels 130 may provide data representing aging effects under different stress conditions. Although only one row and one column of reference pixels 130 are shown in fig. 1, it should be understood that any number of reference pixels may be present. Each reference pixel 130 in the example shown in fig. 1 is fabricated adjacent to a corresponding photosensor 132. The light sensor 132 is used to determine the brightness level emitted by the corresponding reference pixel 130. It should be understood that the reference device (e.g., reference pixel 130) may be a stand-alone device rather than being fabricated on a display having active pixels 104.

Fig. 2 shows one example of a driver circuit 200 for one example reference pixel 130 of fig. 1. The driver circuit 200 of the reference pixel 130 includes a driving transistor 202, an Organic Light Emitting Device (OLED) 204, a storage capacitor 206, a selection transistor 208, and a monitoring transistor 210. The voltage source 212 is connected to the drive transistor 202. As shown in fig. 2, in this example, the driving transistor 202 is a thin film transistor made of amorphous silicon. A select line 214 is connected to the select transistor 208 to activate the driver circuit 200. The voltage programming input line 216 applies a programming voltage to the drive transistor 202. The monitor line 218 monitors the output of the OLED 204 and/or the drive transistor 202. Select line 214 is connected to select transistor 208 and monitor transistor 210. During the read time, select line 214 is pulled high. The program voltage may be applied via a program voltage input line 216. The monitor voltage may be read from a monitor line 218 connected to the monitor transistor 210. The signal to the select line 214 may be sent in parallel with the pixel programming cycle.

The reference pixel 130 may be stressed at a certain current level by applying a constant voltage to the programming voltage input line 216. As will be explained below, the voltage output measured from the monitor line 218 based on the reference voltage applied to the programming voltage input line 216 allows the electrical characteristic data to be determined for the applied stress condition during the runtime of the reference pixel 130. Alternatively, the monitor line 218 and the program voltage input line 216 may be merged into one line (i.e., data/Mon) to perform both programming and monitoring functions through the single line. The output of the light sensor 132 allows the optical characteristic data to be determined for stress conditions during the runtime of the reference pixel 130.

According to an exemplary embodiment, in the display system 100 of FIG. 1, the brightness of each pixel (or sub-pixel) is adjusted based on the aging of at least one pixel to maintain a substantially uniform display over the operating life of the system (e.g., 75000 hours). Non-limiting examples of display devices that include display system 100 include mobile phones, digital cameras, personal Digital Assistants (PDAs), computers, televisions, portable video players, global Positioning Systems (GPS), and the like.

As the OLED material of the active pixel 104 ages, the voltage required to maintain a given level of constant current in the OLED increases. To compensate for the electrical aging of the OLED, the memory 118 stores a compensation voltage required for maintaining a constant current for each active pixel. It also stores data in the form of characteristic dependence curves for different stress conditions, which the controller 112 uses to determine compensation voltages to modify the programming voltage used to drive each OLED of the active pixels 104 to properly display the desired output level of brightness by increasing the OLED current and thereby compensating for the OLED's optical aging. In particular, the memory 118 stores a plurality of predefined characteristic-related curves or functions representing the degradation of the luminance efficiency of OLEDs operating under different predetermined stress conditions. The different predetermined stress conditions generally represent different types of stress or operating conditions that the active pixel 104 may be subjected to during the lifetime of the pixel. The different stress conditions may include different levels of constant current demand from low to high, constant brightness demand from low to high, or a mixture of more than two stress levels. For example, the stress level may be a stress level at a certain current for a certain percentage of time and a stress level at another current for another percentage of time. Other stress levels may be specialized stress levels, for example, levels representing average streaming video (average streaming video) displayed on the display system 100. Initially, baseline electrical characteristics and baseline optical characteristics of a reference device, such as reference pixel 130, under different stress conditions are stored in memory 118. In this example, the baseline electrical characteristic and the baseline optical characteristic of the reference device are measured from the reference device immediately after the reference device is fabricated.

Each such stress condition may be applied to a set of reference pixels (e.g., reference pixels 130) by: maintaining a constant current in the reference pixel 130 for a period of time; maintaining a constant brightness of the reference pixel 130 for a period of time; and/or varying the current in the reference pixel or the brightness of the reference pixel at different predetermined levels and predetermined intervals over a period of time. The current or brightness level produced in the reference pixel 130 may be, for example, a high value, a low value, and/or an average value as desired for a particular application of the display system 100. For example, applications such as computer monitors require high values. Similarly, the period of time that the current or brightness level is generated in the reference pixel may depend on the particular application of the display system 100.

It is contemplated that different predetermined stress conditions are applied to different reference pixels 130 during operation of the display system 100 in order to achieve the same aging effect under each predetermined stress condition. In other words, a first predetermined stress condition is applied to a first set of reference pixels, a second predetermined stress condition is applied to a second set of reference pixels, and so on. In this example, the display system 100 has multiple sets of reference pixels 130 that are stressed under 16 different stress conditions in the range of low current values to high current values for the pixels. Thus, there are 16 different sets of reference pixels 130 in this example. Of course, a greater or lesser number of stress conditions may be employed depending on factors such as the desired accuracy of the compensation, the physical space in the peripheral region 106, the amount of processing power available, and the amount of memory used to store the characterization correlation curve data.

By continuously subjecting the reference pixel or group of reference pixels to a stress condition, the components of the reference pixel age according to the operating conditions of the stress condition. When a stress condition is applied to the reference pixels during operation of the system 100, the electrical and optical characteristics of the reference pixels are measured and evaluated to obtain data for determining correction curves that are used to compensate for aging of the active pixels 104 in the array 102. In this example, the optical and electrical properties are measured once per hour for each set of reference pixels 130. Thus, the corresponding characteristic correlation curve is updated for the measured characteristic of the reference pixel 130. Of course, these measurements may be made over a shorter period of time or a longer period of time, depending on the accuracy desired for the aging compensation.

Generally, the luminance of the OLED 204 has a direct linear relationship with the current applied to the OLED 204. The optical properties of an OLED can be expressed as:

L＝O*I

in this formula, the luminance L is a result of multiplying the current I by a coefficient O based on the characteristics of the OLED. As the OLED 204 ages, the coefficient O decreases, and thus the brightness decreases at a constant current value. Thus, the brightness measured at a given current can be used to determine the aging-induced characteristic change of the coefficient O of a particular OLED 204 at a particular time for a predetermined stress condition.

The measured electrical characteristic represents the relationship between the voltage provided to the drive transistor 202 and the current generated thereby in the OLED 204. For example, the change in voltage required to achieve a constant current level in the OLED of a reference pixel can be measured with a voltage sensor or a thin film transistor such as monitor transistor 210 in FIG. 2. The required voltage generally increases as the OLED 204 and the drive transistor 202 age. The required voltage has a power law relationship with the output current as shown in the following equation.

I＝k*(V-e) ^a

In this formula, the current I is determined by a constant k multiplied by the input voltage V minus a coefficient e, which represents the electrical characteristics of the driving transistor 202. Thus, the voltage and current I have a power law relationship of the variable a. As transistor 202 ages, the coefficient e increases, thereby requiring a greater voltage to produce the same current. Thus, the current measured from the reference pixel may be used to determine the value of the coefficient e for a particular reference pixel at a particular time for the stress condition applied to the reference pixel.

As described above, the optical characteristic O represents the relationship between the luminance of the OLED 204 of the reference pixel 130 in fig. 2 measured by the light sensor 132 and the current in the OLED 204. The measured electrical characteristic e represents the relationship between the applied voltage and the resulting current. The change in brightness of reference pixel 130 at a constant current level relative to a reference optical characteristic may be measured by a light sensor, such as light sensor 132 in fig. 1, when a stress condition is applied to the reference pixel. The change in the electrical characteristic from the reference electrical characteristic can be measured from the monitoring line to determine the current output. During operation of the display system 100, the stress condition current level is continuously applied to the reference pixel 130. When a measurement is desired, the stress condition current is removed and select line 214 is activated. A reference voltage is applied and the resulting brightness level is obtained from the output of the light sensor 132, and the output voltage is measured from the monitor line 218. The data thus obtained is compared with previous optical and electrical data to determine the change in current output and brightness output caused by aging for a particular stress condition and to update the characteristics of the reference pixel under that stress condition. The characteristic correlation curve is updated using the updated characteristic data.

Then, a characteristic correlation curve (or function) over time is determined for a predetermined stress condition by using the electrical and optical characteristics measured from the reference pixels. The characteristic correlation curve provides a quantifiable relationship between expected electrical aging and optical degradation for a given pixel operating under this stress condition. More particularly, each point on the characteristic correlation curve determines a correlation between the optical and electrical characteristics of the OLED of a given pixel under the stress condition at a given time of measurement of the reference pixel 130. The controller 112 can then use this characteristic to determine an appropriate compensation voltage for the active pixel 104 that has aged under the same stress conditions as applied to the reference pixel 130. In another example, the reference optical characteristic may be measured from the base OLED device periodically while measuring the optical characteristic of the OLED of the reference pixel. The base OLED device is not stressed or is stressed at a known and controlled rate. This will eliminate any environmental impact on the reference OLED characteristics.

Due to manufacturing processes and other factors known to those skilled in the art, each reference pixel 130 of display system 100 may not have uniform characteristics, which results in different light emission properties. In one technique, the values of the electrical characteristic and the values of the luminance characteristic obtained by the set of reference pixels under a predetermined stress condition are averaged. A better expression of the influence of stress conditions on the average pixel is obtained by: a set of reference pixels 130 is stressed and a polling averaging (polling) technique is applied to avoid defects, measurement noise, and other problems that may arise during stressing of the reference pixels. For example, error values (e.g., error values determined due to noise or failed reference pixels) may be removed by averaging. This technique may have predetermined brightness levels and electrical characteristics that must be met before those values are included in the averaging. Additional statistical regression techniques may also be used to provide significantly different electrical and optical property values than other measurements with less weight for the reference pixel under a given stress condition.

In this example, each stress condition is applied to a different set of reference pixels. The optical and electrical characteristics of the reference pixels are measured and a round robin averaging technique and/or a statistical regression technique is employed to determine the different characteristic correlation curves corresponding to each stress condition. The different characteristic correlation curves are stored in the memory 118. Although this example uses a reference device to determine the correlation curve, the correlation curve may be determined in other ways, such as based on historical data or predetermined by the manufacturer.

During operation of the display system 100, the reference pixels 130 of each group may be subjected to a respective stress condition, and the characterization correlation curves initially stored in the memory 118 may be updated by the controller 112 to reflect data obtained from the reference pixels 130 subjected to the same external conditions as the active pixels 104. Thus, the characterization correlation curve for each active pixel 104 may be adjusted based on measurements of the electrical and brightness characteristics of the reference pixels 130 during operation of the display system 100. Thus, the electrical and brightness characteristics under each stress condition are stored in the memory 118 and updated during operation of the display system 100. The storage of data may be a piecewise linear model. In this example, such a piecewise linear model has 16 coefficients, the 16 coefficients being updated when measuring the voltage and brightness characteristics of the reference pixels 130. Alternatively, the curve may be determined and updated by using linear regression or by storing the data in a look-up table in memory 118.

Generating and storing a characterization correlation curve for each possible stress condition is impractical because a large amount of resources (e.g., memory storage, processing power, etc.) would be required. The disclosed display system 100 overcomes this limitation by: a discrete number of characterization correlation curves under predetermined stress conditions are determined and stored, and those predefined characterization correlation curves are then combined using a linear or non-linear algorithm to synthesize a compensation factor for each pixel 104 of the display system 100 according to the particular operating conditions of each pixel. As mentioned above, in this example there are 16 different ranges of predetermined stress conditions, and therefore 16 different characteristic correlation curves are stored in the memory 118.

For each pixel 104, the display system 100 analyzes the stress condition being applied to that pixel 104 and determines a compensation factor using an algorithm and based on the pre-defined characterization correlation curve of the panel pixel and the measured electrical aging. The display system 100 then provides a voltage to the pixel based on the compensation factor. Thus, the controller 112 determines the stress of the particular pixel 104 and determines the closest two predetermined stress conditions for the stress condition of the particular pixel 104 and the accompanying characteristic data obtained from the reference pixels 130 under these predetermined stress conditions. Thus, the stress condition of the active pixel 104 falls between the low predetermined stress condition and the high predetermined stress condition.

For ease of disclosure, the following examples of linear and non-linear formulas for combining characteristic correlation curves are described by two such predefined characteristic correlation curves; however, it should be understood that any other number of predefined characteristic correlation curves may be utilized in the exemplary technique for combining characteristic correlation curves. The two exemplary characterization correlation curves include a first characterization correlation curve determined for a high stress condition and a second characterization correlation curve determined for a low stress condition.

The ability to use different characteristic correlation curves for different levels can provide accurate compensation for active pixels 104 that are subject to stress conditions that are different from the predetermined stress conditions applied to reference pixels 130. Fig. 3 is a graph showing different stress conditions of the active pixel 104 over time, showing the luminance levels emitted over time. During the first time period, the luminance of the active pixel is represented by trace 302, trace 302 showing luminance at 300 and 500 nits (cd/cm) ² ) In the meantime. Thus, is applied during trace 302The stress conditions applied to the active pixels are relatively high. In the second time period, the brightness of the active pixels is represented by trace 304, trace 304 showing a brightness between 300 and 100 nits. Thus, the stress condition during the trace 304 is lower than the stress condition for the first time period, and the aging effect of the pixel during this period is different from the aging effect under the high stress condition. In the third time period, the brightness of the active pixel is represented by trace 306, which trace 306 shows a brightness between 100 and 0 nits. The stress condition during this period is lower than the stress condition for the second period of time. During a fourth time period, the brightness of the active pixel is represented by trace 308, trace 308 showing a return to a higher stress condition based on a higher brightness between 400 and 500 nits.

For a particular stress condition of each active pixel 104, a limited number of reference pixels 130 and a corresponding limited number of stress conditions may require the use of an average or a continuous (moving) average. For each pixel, the specific stress condition may be mapped as a linear combination of characteristic correlation curves from the plurality of reference pixels 130. The combination of the two characteristic curves in the predetermined stress condition enables an accurate compensation of all stress conditions occurring between these stress conditions. For example, two reference characterization correlation curves under high and low stress conditions can determine a close characterization correlation curve for an active pixel having a stress condition in between the two reference curves. The controller 112 uses a weighted moving average algorithm (weighted moving average algorithm) to combine the first and second reference profile correlation curves stored in the memory 118. Stress condition St (t) of an active pixel at a certain time _i ) Can be expressed as:

St(t _i )＝(St(t _i-1 )*k _avg +L(t _i ))/(k _avg +1)

in this formula, st (t) _i-1 ) Is the stress condition at the previous time, k _avg Is the moving average constant. L (t) _i ) Is the measured brightness of the active pixel at that certain time, which can be determined by the following equation:

in this formula, L _peak Is the maximum brightness allowed by the design of the display system 100. Variable g (t) _i ) Is the gray level at the time of measurement, g _peak Is the highest gray value used (e.g., 255) and gamma is the gamma constant. A weighted moving average algorithm using characteristic correlation curves for predetermined high and low stress conditions may determine the compensation factor K via the following equation _comp ：

K _comp ＝K _high f _high (ΔI)+K _low f _low (ΔI)

In this formula, f _high Is a first function corresponding to a characteristic correlation curve for a high predetermined stress condition, and f _low Is a second function corresponding to a characteristic correlation curve for a low predetermined stress condition. Δ I is the change in current in the OLED at a fixed voltage input, which shows the change caused by aging effects (electrical degradation) measured at a specific time. It will be appreciated that the change in current may be replaced by a change in voltage at a fixed current Δ V. K _high Is a weighted variable of the characteristic dependence curve assigned to the high stress condition, and K _low Is the weight assigned to the characteristic correlation curve for the low stress condition. The weighting variable K can be determined according to the following formula _high And K _low ：

K _high ＝St(t _i )/L _high

K _low ＝1-K _high

Here, L _high Is the brightness associated with high stress conditions.

The change in voltage or current in the active pixel at any time during operation is indicative of an electrical characteristic, while the change in current as part of a high or low stress condition is indicative of an optical characteristic. In this example, the luminance under high stress conditions, the peak luminance, and the average compensation factor (a function of the difference between the two characteristic correlation curves) K are measured _avg Stored in memory 118 for determining each active pixelA compensation factor. Additional variables are stored in memory 118, including, but not limited to, the gray scale value for maximum brightness allowed by display system 100 (e.g., gray scale value 255). In addition, the average compensation factor K may be empirically determined from data obtained during application of a stress condition to a reference pixel _avg 。

Thus, the relationship between optical degradation and electrical aging of any pixel 104 in the display system 100 may be adjusted to avoid errors associated with differences in the characteristic correlation curves (divergences) caused by different stress conditions. The number of stored characterization correlation curves may also be minimized to a number that ensures that the averaging technique is accurate enough for the required level of compensation.

Compensation factor K _comp Can be used to compensate for OLED light efficiency aging by adjusting the programming voltage of the active pixel. Another technique for determining an appropriate compensation factor for stress conditions on an active pixel may be referred to as dynamic moving averaging (dynamic moving averaging). The dynamic moving average technique includes: varying the moving average coefficient K over the lifetime of the display system 100 _avg To compensate for differences between the two characteristic correlation curves under different predetermined stress conditions, thereby preventing distortion of the display output. As the OLED of the active pixel ages, the difference between the two characteristic correlation curves increases under different stress conditions. Thus, K may be increased during the lifetime of display system 100 _avg To avoid sharp transitions between the two curves of an active pixel having a stress condition that falls between two predetermined stress conditions. K can be adjusted using the measured current change Δ I _avg To a value to improve the performance of the algorithm used to determine the compensation factor.

In another technique for improving the performance of the compensation process, known as event-based moving averaging, the system is reset after each aging stage. This technique further improves the extraction of the characteristic correlation curve for the OLED of each active pixel 104. Resetting the display system 10 after each burn-in phase (or after the user turns the display system 100 on or off)0. In this example, the compensation factor K is determined by the following formula _comp ：

K _comp ＝K _{comp_evt} +K _high (f _high (ΔI)-f _high (ΔI _evt ))+K _low (f _low (ΔI)-f _low (ΔI _evt ))

In this formula, K _{comp_evt} Is a compensation factor calculated at a previous time, and Δ I _evt Is the change in OLED current during a previous time at a fixed voltage. As with other compensation determination techniques, changes in current can be replaced by changes in OLED voltage at a fixed current.

Fig. 4 is a graph 400 illustrating different characterization correlation curves based on different techniques. Graph 400 compares the change in percentage of optical compensation with the change in voltage of the OLED of the active pixel required to produce a given current. As shown in graph 400, the high stress predetermined characteristic correlation curve 402 deviates from the low stress predetermined characteristic correlation curve 404 at greater changes in voltage to reflect aging of the active pixels. The set of points 406 represents a correction curve for current compensation of the active pixel determined by a moving averaging technique and at different voltage variations according to the predetermined characteristic correlation curves 402 and 404. As the change in voltage for reflecting the aging increases, the transition of the correction curve 406 has a sharp transition between the low stress characteristic-related curve 404 and the high stress characteristic-related curve 402. Set of points 408 represents a characteristic correlation curve determined by a dynamic moving averaging technique. The set of points 410 represent compensation factors determined by an event-based moving averaging technique. One of the above techniques may be used to improve the compensation for OLED efficiency degradation based on OLED characteristics.

As described above, the electrical characteristics of the first set of sample pixels are measured. For example, the electrical characteristics of each pixel in the first set of sample pixels may be measured by a Thin Film Transistor (TFT) connected to each pixel. Alternatively, for example, the optical characteristic (e.g., brightness) may be measured by a photosensor provided for each of the sample pixels of the first group. The amount of change required for the luminance of each pixel can be extracted from the drift of the voltage of more than one pixel. This may be achieved by a series of calculations for determining a shift in the voltage or current supplied to a pixel and/or a correlation between the brightness of the luminescent material in the pixel.

The above-described method for extracting characterization correlation curves to compensate for aging of pixels in an array may be performed by a processing device, such as the processing device of controller 112 in fig. 1 or other such devices, which may be conveniently implemented in one or more general purpose computer systems, microprocessors, digital signal processors, microcontrollers, application Specific Integrated Circuits (ASICs), programmable Logic Devices (PLDs), field Programmable Logic Devices (FPLDs), field Programmable Gate Arrays (FPGAs), etc., programmed according to the teachings described and illustrated herein, as will be appreciated by those skilled in the computer, software, and networking arts.

Additionally, more than two computing systems or devices may be substituted for any of the controllers described herein. Thus, the principles and advantages of distributed processing, such as redundancy, replication, etc., can also be implemented, as needed, to increase the robustness and performance of the controller described herein.

Operations for compensating an example characterization correlation curve of an aging method may be performed by machine readable instructions. In these examples, the machine-readable instructions comprise an algorithm that is executed by: a (a) processor, (b) controller, and/or (c) one or more other suitable processing devices. The algorithm may be implemented as software stored on a tangible medium such as a flash memory, a CD-ROM, a floppy disk, a hard drive, a digital video (versatile) disk (DVD), or other memory devices, but persons of ordinary skill in the art will readily appreciate that the entire algorithm and/or parts thereof could alternatively be executed by a device other than a processor and/or implemented as firmware or dedicated hardware in a well-known manner (e.g., it may be implemented by an Application Specific Integrated Circuit (ASIC), a Programmable Logic Device (PLD), a Field Programmable Logic Device (FPLD), a Field Programmable Gate Array (FPGA), discrete logic, etc.). For example, any or all of the components of the characterization correlation curve used to compensate for aging methods can be implemented by software, hardware, and/or firmware. Further, some or all of the depicted machine readable instructions may be implemented manually.

Fig. 5 is a flow chart of a process for determining and updating a characterization correlation curve for a display system (e.g., display system 100 of fig. 1). The stress conditions are selected to provide a sufficient reference (500) for correlating the range of stress conditions for the active pixels. Then, a set of reference pixels is selected for each stress condition (502). Then, the reference pixels of each group corresponding to the stress condition are stressed at each stress condition, and the optical and electrical characteristics of the fiducials are stored (504). The brightness level of each pixel in each group is measured and recorded at periodic intervals (506). The luminance characteristic is then determined by averaging the measured luminance of each pixel in the group of pixels under each stress condition (508). Electrical characteristics of each pixel in each group are determined (510). An average value for each pixel in the group is determined to determine an average electrical characteristic (512). The average luminance characteristic and the average electrical characteristic of each group are then used to update a characteristic correlation curve (514) for the corresponding predetermined stress condition. Once the correlation curve is determined and updated, the controller may use the updated characteristic correlation curve to compensate for aging effects of active pixels subjected to different stress conditions.

Referring to fig. 6, a flow chart of a process for determining a compensation factor for an active pixel at a given time using an appropriate predetermined characteristic correlation curve for the display system 100 as obtained in the process of fig. 5 is shown. The brightness emitted by the active pixel is determined based on the maximum brightness and the programming voltage (600). The stress condition of a particular active pixel is measured based on the previous stress condition, the determined brightness, and the average compensation factor (602). The appropriate predetermined stress characteristic correlation curve is read from memory (604). In this example, the two characteristic correlation curves correspond to predetermined stress conditions, wherein the measured stress condition of the active pixel falls between these predetermined stress conditions. The controller 112 then determines coefficients according to each predetermined stress condition by using the current or voltage variations measured from the active pixels (606). The controller then determines the modified coefficients to calculate and add a compensation voltage to the programming voltage of the active pixel (608). The determined stress condition is stored in memory (610). The controller 112 then stores the new compensation factor, which may then be employed to modify the programming voltage of the active pixel during each frame period following the measurement of the reference pixel 130 (612).

The OLED efficiency degradation can be calculated based on a correlation curve between the OLED electrical change and the efficiency degradation (e.g., the correlation curve in fig. 7). Here, the change in the electrical parameter of the OLED is detected and this value is used to extract the efficiency degradation from the curve. The pixel current can then be adjusted accordingly to compensate for the degradation. The main challenge is that the correlation curve is a function of the stress condition. Therefore, in order to achieve a more accurate compensation, one need is to take into account the effects of different stress conditions. In one approach, the stress condition of each pixel (group of pixels) is used to select among different correlation curves to extract the appropriate efficiency loss for each particular case. A number of methods for determining the stress condition will now be described.

First, a stress history for each pixel (pixel group) can be created. Simply, the stress history may be a moving average of the stress conditions. To improve the computational accuracy, a weighted stress history may be used. Here, as in the example depicted in fig. 8, the influence of each stress may have a different weight based on the stress intensity or period. For example, the impact of low intensity stress is small in the selection of the OLED correlation curve. Thus, a curve with a small weight at a small intensity, such as the curve in fig. 8, may be used. The stress history may also be computed using sub-sampling to reduce memory transfer activity. In one case, the stress history may be assumed to be low frequency in time. In this case, the pixel condition for each frame need not be sampled. The sampling rates of different applications may be modified based on the content frame rate. Here, only a small number of pixels are selected to obtain an updated stress history during each frame.

In another case, the stress history may be assumed to be spatially low frequency. In this case, it is not necessary to sample all pixels. Here, the stress history is computed using a subset of pixels, and then the stress history for all pixels may be computed using interpolation techniques.

In another case, a low sampling rate in time and a low sampling rate in space may be combined.

In some cases, the memory and computing modules required for stress history may not be included. Here, as shown in fig. 9A and 9B, the rate of change of the OLED electrical parameters can be used to extract the stress condition. FIG. 9A shows Δ V under low, medium and high stress conditions _OLED Time and 9B shows the time dependence of the rate of change under the same stress conditions.

As shown in fig. 10, the rate of change of the electrical parameter can be used as an indicator of the stress condition. For example, as shown in FIG. 10, the rate of change of the electrical parameter based on the change of the electrical parameter can be modeled or extracted experimentally for different stress conditions. The rate of change can also be used to extract a stress condition based on a comparison of the measured change to the rate of change of the electrical parameter. Here, functions established for the change and rate of change of the electrical parameter are used. Alternatively, the stress condition, the correlation curve, and the measured variation parameter may be used.

FIG. 11 is a flow chart of a process for compensating for OLED efficiency degradation based on a measurement of the change and rate of change of OLED electrical parameters. In this process, the change in the OLED parameter (e.g., OLED voltage) is extracted in step 1101, and then the rate of change of the OLED parameter is calculated based on the previously extracted value in step 1102. Next, step 1103 identifies a stress condition using the change in the parameter and the rate of change. Finally, step 1104 calculates the efficiency degradation from the stress condition, the measured parameter, and the correlation curve.

As described in fig. 12, the OLED efficiency degradation can be compensated by using a correlation curve of the OLED electrical change (current or voltage) and the efficiency degradation. The correlation curve may vary due to process variations. In one embodiment, a test OLED may be used in each display, and the curve for each display is extracted after manufacture or during display operation. In the case of smaller displays, the test OLED devices can be mounted on a substrate and used to extract curves after fabrication.

FIG. 13 is a flow chart of a process for extracting a correlation curve from a test device after a line is taken down, during a display operation, or a combination of both. In this case, the curves extracted in the factory are stored for aging compensation. During display operations, the curves can be updated with additional data based on measurements of test devices in the display. However, since the extraction process may take time, the set of curves may be measured in advance and placed in a library. Here, the test devices are aged at a predetermined aging level (generally higher than normal) to extract some aging characteristics (and/or measure their current-voltage-luminance IVL) in a short period of time. Thereafter, the extracted aging characteristic is used to find an appropriate curve from the curve library having a similar or close aging characteristic.

In fig. 13, a test device is added to the substrate inside or outside the display area in a first step 1301. Next, the test device is measured to extract a correlation curve in step 1302. A relevance curve for the display on the substrate is calculated based on the measured curve in step 1303. The curves for each display are stored in step 1304 and then used to compensate for display aging in step 1305. Alternatively, the test device may be measured during the display operation in step 1306. Next, the correlation curve is updated based on the measured result in step 1307. If necessary, curves are derived in step 1308 and the display is compensated in step 1309 based on these curves.

The following are some examples of procedures for finding an appropriate curve from a library:

(1) The curve with the closest aging characteristic (and/or IVL characteristic) is selected.

(2) Samples in the library having characteristics closer to the test sample are used and a curve of the display is created. Here, a weighted average may be used in which the weight of each curve is determined based on the error between their aging characteristics.

(3) If the error between the closest set of curves in the library and the test device is greater than a predetermined threshold, the test device can be used to create new curves and add them to the library.

Fig. 14 is a flow chart of a process for accounting for process differences between or within substrates. In a first step 1401 the test device is added to the substrate inside or outside the display area, or the test device may be the display itself. The test device is then tested for a predetermined burn-in level in step 1402 to extract the burn-in characteristics and/or to measure the IVL characteristics of the test device. A set of samples in the library of correlation curves having burn-in or test characteristics closest to the test device is found in step 1403. Next, it is determined whether the error between the IVL and/or aging characteristics is less than a threshold in step 1404. If the answer is in the affirmative, then the curves in the library are used to calculate a correlation curve for the display in the substrate in step 1405. If the answer in step 1404 is negative, then the test device is used to extract a new correlation curve in step 1406. Next, in step 1407, the curves are used to calculate the relevance curves for the displays in the substrate, and these new curves are added to the library in step 1408.

Semiconductor devices (e.g., OLEDs) may age differently under different stress conditions as well as environmental conditions (e.g., temperature, brightness, etc.). Moreover, some rare stress conditions may drive the device to aging conditions that are different from normal conditions. For example, extremely high stress conditions may physically damage the device (e.g., affect contacts or other layers). In this case, identification of the compensation curve may require additional information that can be obtained from other devices in the pixel (e.g., transistors or sensors) from the rate of change in device characteristics (e.g., threshold voltage shift or mobility change) or by using changes in multi-device parameters (multi-device parameters) to identify stress conditions. Where other devices are used, the rate of change of the parameter of the other device and/or the rate (or absolute value) of change of the parameter of the other device relative to the rate (or absolute value) of change of the parameter of the device may be used to identify the aging condition. For example, at higher temperatures, TFTs and OLEDs become faster, and thus the rate of change can be an indicator of temperature change as the TFT or OLED ages.

15A and 15B are flow diagrams illustrating a process of identifying a stress condition of a device based on or based on a comparison of a rate of change or an absolute value of at least one parameter of at least one device with a rate of change or an absolute value of at least one parameter of at least one other device. The identified stress conditions are used to select an appropriate compensation curve and/or parameters of the extraction device based on the identified stress conditions. A compensation parameter of the device is calculated using the selected compensation curve, and the input signal is compensated based on the calculated compensation parameter.

In fig. 15A, in a first step 1501a, a rate of change or absolute value of at least one parameter of at least one device (e.g., OLED) is examined, and then in step 1502a stress condition is identified based on the rate of change or absolute value. Next, in step 1503a, an appropriate compensation curve for the device is selected and/or parameters of the device are extracted based on the identified stress condition. The compensation parameters for the device are calculated using the selected compensation curve in step 1504a and then the input signal is compensated based on the calculated compensation parameters in step 1505 a.

In fig. 15B, in a first step 1501B, the rate of change or absolute value of at least one parameter of at least one device (e.g., OLED) is compared to the rate of change or absolute value of at least one parameter of at least one other device. Next, stress conditions are identified based on the comparison in step 1502b, and an appropriate compensation curve for the device is selected or parameters of the device are extracted based on the identified stress conditions in step 1503 b. In step 1504b, the compensation parameters for the device are calculated using the selected compensation curve, and then in step 1505b the input signal is compensated based on the calculated compensation parameters.

In another embodiment, the rate of change of different parameters of a device may be viewed to identify stress conditions. For example, in the case of an OLED, voltage (or current) shifts at different current levels (or voltage levels) can identify stress conditions. Fig. 16 is an example of the IV characteristics of an OLED under three different conditions (i.e., initial condition, stressed at 27 ℃, or stressed at 40 ℃). It can be seen that the properties change significantly as the stress condition changes.

Fig. 17 and 18 are flow diagrams of processes for equalizing pixels in a light emitting display panel having an array of pixels that include semiconductor devices that age under different environmental and stress conditions. Fig. 17 shows a process for achieving initial equalization of pixels, and fig. 18 shows a process for achieving equalization of pixels after a period of use.

In the process shown in fig. 17, at least one pixel parameter (pixel information) is extracted from the light-emitting display panel in step 1701. In step 1702, a stress pattern is created using these parameters. These stress patterns are applied to the panel in step 1703, and the pixel parameters are monitored and updated by extracting them from the stressed pixels in step 1704. In step 1705, it is determined whether the pixel parameters extracted from the stressed pixel are within a preselected range, and if the answer is negative, steps 1702-1705 are repeated. This process continues until a positive answer is generated in step 1705, meaning that the pixel parameters extracted from the stressed pixel are within the preselected range, and thus the pixel is returned to normal operation.

The stress pattern may include duration and stress level. In one embodiment of the invention, pixel parameters are monitored on-line during stress to ensure that the pixel parameters do not fall outside of specified ranges. In another embodiment of the invention, the parameters of the selected pixels or some of the reference pixels are monitored online during the stress. In another embodiment of the invention, the pixel is stressed for a period of time and then the pixel parameters are extracted. Thereafter, the pixel parameters are updated, and the stress pattern and the point in time (timing) can be updated with new data including the new pixel parameters and the rate of change. For example, if the rate of change is fast, the stress interval is made smaller to avoid exceeding a specified range of pixel parameters.

The setting of the parameters of the pixels may differ between the parameters across the panel. In another embodiment, it may be a specified value.

In one example, the pixel information (or parameter) may be a threshold voltage of the driving TFT. Here, the stress condition of each pixel is defined based on its threshold voltage. In another example, the pixel parameter may be a voltage (or brightness uniformity) of the light emitting device.

The pixel information may be extracted by different means. One method is by power supply. In another case, the pixel parameters may be extracted by the monitor lines.

In fig. 18, the pixel parameters are extracted after the use period. For example, the extraction may be triggered by a user, a timer, or a specified operating condition (e.g., in a charging mode). In step 1801, during the use period, a stress history for the pixel is created, and in step 1802, the pixel parameters are extracted after the use period. The stress history may include stress levels during operation and stress time points. In another embodiment, the stress history may be an average stress condition of the pixel during the usage period.

In step 1803, a stress pattern is generated based on the extracted pixel parameters and the stress history. Next, at step 1804, the pixels are stressed according to the generated stress pattern. In step 1805, the parameters of the stressed pixels are monitored and updated by extracting pixel parameters from the stressed pixels. In step 1806, it is determined whether the pixel parameters extracted from the stressed pixel are within a pre-selected range, and if the answer is negative, the stress history of the pixel is updated in step 1807, and steps 1803-1806 are then repeated. This process continues until a positive answer is generated in step 1806, meaning that the pixel parameters extracted from the stressed pixel are in the pre-selected range and thus the pixel is returned to normal operation.

In one example, pixels are assigned to different categories based on stress history, and then stressed with all other categories to which the pixels are not assigned. At this point, the pixel parameters are monitored in a manner similar to that described above to ensure that they are not out of specification.

In another example, the stress history does not have temporal information, and changes in pixel parameters can be used to identify stress levels and points in time. For example, in one case, for the stress pattern, a shift in electrical characteristics of the light emitting device may be used to extract the stress condition of each pixel.

In yet another embodiment, for each pixel, a correlation curve between the pixel parameter and its optical performance can be used to extract the stress condition. In the case of electrical characteristics of the light emitting device, the correlation curve may be used to find the worst case of efficiency degradation. Then, the efficiency difference between each pixel and the worst case may be determined. Thereafter, the corresponding change in the electrical characteristics of the light emitting device of each pixel may be calculated to minimize the difference in efficiency between the pixel and the worst case. The pixels are then stressed and their pixel parameters (e.g., electrical characteristics of the light emitting device) are monitored to achieve the calculated offset. Similar operations may be used for other pixel parameters.

While particular embodiments, aspects and applications of the present invention have been shown and described, it is to be understood that the invention is not limited to the precise configuration and arrangement disclosed herein and that various modifications, changes and variations may be apparent from the foregoing descriptions without departing from the spirit and scope of the invention as defined in the appended claims.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. patent application 14/590,105 filed on 6/1/2015, which prior application is incorporated herein by reference in its entirety.

Claims (20)

1. A method for equalizing a pixel circuit in an array of pixel circuits, the pixel circuit comprising semiconductor devices that age differently under different environmental and stress conditions, the method comprising:

a) During a use period, creating a stress history of the pixel circuit using a controller, the stress history comprising data relating to a history of stress to which the pixel circuit has been subjected;

b) Extracting pixel parameters from the pixel circuit array using at least one of a voltage sensor, a current sensor, and a light sensor during or after the use period;

c) Updating, using the controller, a stress pattern of the pixel circuit array based on the extracted pixel parameters and the stress history;

d) Subjecting the pixel circuit to stress according to the stress pattern;

e) Extracting the pixel parameter from the stressed pixel circuit using at least one of the voltage sensor, the current sensor, and the light sensor;

f) Determining, using the controller, whether the pixel parameter extracted from the stressed pixel circuit is within a preselected range; and is

When the pixel parameter is not within a pre-selected range: updating the stress history and repeating steps c) through f); and is

Returning the pixel circuit array to normal operation when the pixel parameter is within a preselected range.

2. A method according to claim 1, wherein the pixel parameter comprises a threshold voltage of a drive transistor in each active pixel circuit.

3. The method of claim 1, wherein the pixel parameter comprises a brightness level.

4. The method of claim 1, wherein the pixel parameter comprises a current output of each pixel circuit.

5. The method of claim 1, wherein the pixel parameters comprise differences between the pixel parameters across the pixel circuit array.

6. The method of claim 1, wherein step d) comprises applying stress to the pixel circuit using a current source.

7. The method of claim 1, wherein the stress history comprises stress times and stress levels.

8. The method of claim 1, wherein the stress history comprises an average stress condition of individual pixel circuits during the period of use.

9. The method of claim 1, wherein step e) is performed simultaneously with step d).

10. The method of claim 1, wherein step e) is performed after step d).

11. A system, comprising:

a plurality of pixel circuits for displaying an image, the pixel circuits each including a semiconductor device that ages differently under different environmental and stress conditions;

a controller connected to the plurality of pixel circuits and configured to:

a) Creating a stress history for the plurality of pixel circuits during or after a use period, the stress history comprising data relating to a history of stress to which the plurality of pixel circuits have been subjected;

b) After the use period, controlling to extract pixel parameters from the plurality of pixel circuits;

c) Updating a stress pattern of the plurality of pixel circuits based on the extracted pixel parameters and the stress history;

d) Controlling to apply stress to the plurality of pixel circuits according to the stress pattern;

e) Controlling to extract the pixel parameter from the stressed pixel circuit;

f) Determining whether the pixel parameter extracted from the stressed pixel circuit is within a preselected range; and is

When the pixel parameter is not within a pre-selected range: updating the stress history and repeating steps c) through f); and is

Returning the plurality of pixel circuits to normal operation when the pixel parameter is within a preselected range.

12. The system of claim 11, wherein the pixel parameter comprises a threshold voltage of a drive transistor in at least one of the plurality of pixel circuits; and is

The system further includes a voltage sensor for extracting the threshold voltage.

13. The system of claim 11, wherein the pixel parameters include a brightness level; and is

The system further includes at least one light sensor for extracting the brightness level from at least one of the plurality of pixel circuits.

14. The system of claim 11, wherein the pixel parameter comprises a current output of at least one of the plurality of pixel circuits; and is provided with

The system further includes at least one current sensor for extracting the current output.

15. The system of claim 11, wherein the pixel parameters comprise differences between the pixel parameters across the plurality of pixel circuits.

16. The system of claim 11, further comprising a current source and a read circuit for applying stress to the plurality of pixel circuits.

17. The system of claim 11, wherein the stress history comprises stress times and stress levels.

18. The system of claim 11, wherein the stress history comprises an average stress condition of each pixel circuit during the period of use.

19. The system of claim 11, wherein step e) is performed simultaneously with step d).

20. The system of claim 11, wherein step e) is performed after step d).

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